1. Field of the Invention
The present invention relates to methods and apparatus for permitting the flow of fluids from a plurality of fluid-bearing subterranean intervals or zones into a wellbore and, more particularly, to such methods and apparatus wherein adjacent zones are produced through a single casing within the wellbore.
2. Setting of the Invention
After an oil or gas well has been drilled and casing is cemented in the wellbore, it may be necessary or desirable to perforate the casing at several different levels in order to produce oil or gas from different fluid-bearing zones. Sometimes it may be necessary to produce each zone in isolation from the other; that is, concentric strings of production tubing are suspended in the casing with each string terminating at the level of the different zones. Annular seals or packers are set between the tubing strings to fluidically seal each zone from the others. Thereafter, formation fluids are produced from each zone through its associated tubing for use at the surface.
Often it is desirable to complete a well having a plurality of hydrocarbon-bearing zones through a single string of production tubing. In such cases, a string of production tubing is placed in the wellbore at a level immediately above the uppermost zone from which fluids are to be produced. A packer is used to seal the annulus between the tubing and the wellbore. Thereafter, a commercially available perforating device is lowered through the production tubing to various levels and is activated to perforate the selected zones. Fluids from the perforated formations are then produced through the casing and into the production tubing for use at the surface.
In some cases, it may be necessary to hydraulically fracture one or more zones as part of the completion process. A zone is hydraulically fractured by pumping a propping agent; e.g, sand, into the zone after the casing at that level has been perforated. It is often necessary to isolate a zone from the other zones prior to fracturing since each zone can require different quantities of propping agent at varying pressures for optimum results from the fracturing process. There are at least two known methods for fracturing multiple zones in a wellbore which are to be produced through a single string of production tubing.
In one method, a lower zone is first perforated and then fractured if necessary. Thereafter, a commercially available plug is lowered on a wireline and set in the wellbore above the fractured zone to seal it from the wellbore above the plug. A zone above the plug is then perforated and fractured. After fracturing the zone above the plug, the plug is retrieved from the wellbore or reset at a higher level in the wellbore to fracture another zone. During fracturing of the upper zone, sand settles on the top of the plug and oftentimes prevents its retrieval. The sand can be washed away by lowering tubing into the well and pumping fluids down the tubing to force the sand upwardly in the wellbore between the tubing and the casing. However, lowering such tubing to wash the sand away creates a problem in that the production tubing is opened to atmospheric pressure at the upper end of the well so it can be lowered. This necessitates filling the well with heavy fluids, known as killing the well, to prevent high pressure flow from the formation into the wellbore which could cause a blowout while the tubing is in the wellbore. Killing the well is both expensive and time-consuming and poses a risk in that the formation's ability to produce fluids can be impaired by the absorption of the heavy fluids into the formation.
Another known procedure can be used to fracture a zone in a wellbore having a plurality of zones which are to be produced through a common tubing. In this procedure, a lower zone is fractured if necessary. The wellbore adjacent this zone is filled with sand while an upper zone is perforated and fractured. The sand about the lower zone prevents entry of the fracturing fluid and propping agent (injected into the wellbore during fracturing of the upper zone) from entering the lower zone. After fracturing the upper zone, the production tubing at the surface is opened sufficiently to permit well flow and, it is hoped, to remove the sand used to cover the lower zone along with the well fluids. If this is not so, tubing is lowered to wash away the sand, which necessitates killing the well, an undesirable procedure for other reasons mentioned above. Even if the sand is produced from the well along with the fluids, there can be problems caused by removing sand along with fluids. Such sand can damage chokes, valves, manifolds, and other production equipment.
There exists a need for a method and apparatus for completing a plurality of zones in a wellbore when one of the upper zones is hydraulically fractured. Moreover, there exists a need for such a method and apparatus which can be utilized without killing the well and without the other problems attendant with the above-described prior art methods and apparatus.